The present invention relates to magneto-optical media, such as, magneto-optical disks, magneto-optical tapes, and magneto-optical cards, that are applicable to magneto-optical recording/reproducing apparatuses.
Some time has passed since rewritable optical storage disks were put into practical use as a first application of a magneto-optical storage medium. To delete stored data from such optical storage disks, the temperature of a part of the magneto-optical storage medium is elevated by a light beam which is emitted from a semiconductor laser and focused on that part of the magneto-optical storage medium. And then, a light beam having intensity at which data is not deleted is emitted and focused on the magneto-optical storage medium, and polarization of the reflected light is recognized, so that the stored data is reproduced.
However, in such a magneto-optical storage medium, when the magnetic recording domain has a smaller recording bit diameter and a smaller recording bit interval than the beam spot of the light beam, reproduction characteristics are degraded. This is because an adjacent recording bit enters a beam spot of a light beam focused on a target recording bit, so individual recording bits cannot be distinguished for reproduction.
Japanese Unexamined Patent Publication No. 9-180276/1997 (Tokukaihei 9-180276; published on Jul. 11, 1997; corresponding to U.S. Pat. No. 5,777,953) discloses a magneto-optical storage medium for solving the above problem. The magneto-optical storage medium includes: a magnetic reproduction layer, which exhibits in-plane magnetization at room temperature and changes to perpendicular magnetization at a critical temperature or a higher temperature; a non-magnetic intermediate layer; and a magnetic storage layer, for storing data, which is constituted by a perpendicularly magnetized film.
Further, Japanese Unexamined Patent Publication No. 9-320134/1997 (Tokukaihei 9-320134; published on Dec. 12, 1997; corresponding to U.S. Pat. No. 5,939,187) discloses a magneto-optical storage medium for improving reproduction characteristics. The magneto-optical storage medium includes: a magnetic reproduction layer, which exhibits in-plane magnetization at room temperature and changes perpendicular magnetization at a critical temperature or a higher temperature, an in-plane magnetized layer having a Curie temperature around the foregoing critical temperature, a non-magnetic intermediate layer, and a magnetic storage layer, for storing data, which is constituted by a perpendicularly magnetized film.
Furthermore, Japanese Unexamined Patent Publication No. 8-180486/1996 (Tokukaihei 8-180486; published on Jul. 12, 1996;. corresponding to U.S. Pat. No. 5,659,537) discloses a magneto-optical storage medium including: a magnetic reproduction layer which exhibits perpendicular magnetization; a non-magnetic intermediate layer; and a magnetic storage layer, for storing data, which is constituted by a perpendicularly magnetized film.
According to Tokukaihei 9-180276 and Tokukaihei 9-320134, in the magneto-optical storage medium, the magnetic reproduction layer changes to in-plane magnetization at a critical temperature or a lower temperature. Thus, the magnetic information stored in the magnetic recording domain in the magnetic storage layer is not copied to the reproduction layer, so the information is not reproduced. Meanwhile, the magnetic reproduction layer changes to perpendicular magnetization at a critical temperature or a higher temperature, the information stored in the magnetic recording domain in the magnetic storage layer is copied to the magnetic reproduction layer, and the information stored in the magnetic recording domain is reproduced. Thus, even when an adjacent recording bit enters a beam spot of a light beam focused on the magnetic reproduction layer, individual recording bits can be distinguished from one another in a reproduction process by suitably setting the reproduction power of a light beam and the critical temperature where the magnetic reproduction layer changes to perpendicular magnetization. Consequently, it is possible to achieve magnetic super-resolution reproduction whereby information stored with high density is reproducible.
Also, in the magneto-optical storage medium disclosed in Tokukaihei 8-180486, magnetic information is copied from the magnetic reproduction layer to the magnetic storage layer only in a part where temperature is elevated, and magnetic super-resolution reproduction can be realized similarly to the foregoing.
However, in recent years, optical disks with a larger storage capacity have been in demand. Hence, it has been necessary to form smaller magnetic recording domains in the magnetic storage layer, to copy the magnetic recording domains to the magnetic reproduction layer, and to perform reproduction in a stable manner.
The magnetic storage layers, disclosed in Tokukaihei 9-180276, Tokukaihei 9-320134, and Tokukaihei 8-180486, for use in magneto-optical storage media have a compensation temperature around room temperature, records and holds information, and produces a leaking magnetic flux for copying information from a magnetic recording domain in the magnetic storage layer to the magnetic reproduction layer in a reproduction process. Therefore, the Curie temperature of the magnetic storage layer is set in a range of from 225xc2x0 C. and 275xc2x0 C. to prevent excessive recording power. In this case, in a reproduction process, when temperature is elevated at the center of a light beam spot to around a Curie temperature, the net magnetization of the magnetic storage layer decreases around the center of the light beam spot. Thus, a leaking magnetic flux from the magnetic storage layer grows smaller so as to weaken magnetostatic coupling forces between the magnetic storage layer and the magnetic reproduction layer; hence, the information stored in a magnetic domain is not copied to the magnetic reproduction layer. Consequently, the net magnetization of the magnetic storage layer becomes extremely small according to a temperature increase, so that a reproduction power margin is narrowed, a smaller magnetic recording domain cannot be copied to the magnetic reproduction layer, and stable reproduction is not possible.
An objective of the present invention is to provide a magneto-optical storage medium and a recording method thereof, that can increase the net magnetization of the magnetic storage layer during reproduction and reproduce data stored in a small magnetic recording domain in a stable manner.
A magneto-optical storage medium in accordance with the present invention, to achieve the foregoing objective, includes:
a reproduction layer which exhibits in-plane magnetization at room temperature and changes perpendicular magnetization at a temperature equal to, or higher than, a predetermined temperature;
a storage layer, for storing magnetic information, which is magnetostatically coupled to the reproduction layer; and
a magnetic flux forming layer for producing a leaking magnetic flux so as to be magnetostatically coupled to the reproduction layer, the magnetic flux forming layer being exchange coupled to the storage layer.
According to the magneto-optical storage medium, magnetostatic coupling is established during a reproduction process by the leaking magnetic flux arising from the storage layer and the perpendicular magnetization of the reproduction layer and copies the magnetic information in the storage layer to the reproduction layer. Therefore, magnetic information can be reproduced only from a part of the reproduction layer where temperature is equal to, or higher than, the predetermined temperature. This enables magnetic super-resolution reproduction whereby magnetic information stored with high density is reproducible.
Incidentally, if a storage layer plays dual roles of storing information and of producing a leaking magnetic flux to establish magnetostatic coupling, as is the case with conventional disks, the net magnetization in the storage layer decreases with an increase in the reproduction power during a reproduction process. As a result, the storage layer fails to produce a sufficiently large leaking magnetic flux, which weakens the magnetostatic coupling between the storage layer and the reproduction layer. This disables stable reproduction and results in a narrow reproduction power margin.
In contrast, the magneto-optical storage medium includes a magnetic flux forming layer which is exchange coupled to the storage layer and which also produces a leaking magnetic flux so as to be magnetostatically coupled to the reproduction layer. This enables the magnetic flux forming layer exchange coupled to the storage layer to produce a leaking magnetic flux, despite the fact that the storage layer loses some of its net magnetization with an increase in the reproduction power. Therefore, the magnetostatic coupling established by the leaking magnetic flux arising from the magnetic flux forming layer ensures the copying of the magnetic information in the storage layer to the reproduction layer. Consequently, an increase in the reproduction power does not disturb stable reproduction and does not lead to a narrow reproduction power margin.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.